In a direct injection system of the common-rail type, a high-pressure pump receives a flow of fuel from a tank by means of a low-pressure pump and feeds the fuel to a common rail hydraulically connected to a plurality of injectors. The pressure of the fuel in the common rail must be constantly controlled according to the engine point either by varying the instantaneous flow rate of the high-pressure pump or by constantly feeding an excess of fuel to the common rail and by discharging the fuel in excess from the common rail itself by means of an adjustment valve. Generally, the solution of varying the instantaneous flow rate of the high-pressure pump is preferred, because it displays a much higher energy efficiency and does not cause an overheating of the fuel.
In order to vary the instantaneous flow rate of the high-pressure pump, there has been suggested a solution of the type presented in patent application EP0481964A1 or in U.S. Pat. No. 6,116,870A1 which describe the use of a variable flow rate high-pressure pump capable of feeding the common rail only with the amount of fuel needed to maintain the fuel pressure within the common rail equal to the desired value; specifically, the high-pressure pump is provided with an electromagnetic actuator capable of varying the flow rate of the high-pressure pump instant-by-instant by varying the closing instant of an intake valve of the high-pressure pump itself.
Alternatively, in order to vary the instantaneous flow rate of the high-pressure pump, it has been suggested to insert a flow adjusting device upstream of the pumping chamber comprising a continuously variable section bottleneck which is controlled according to the required pressure within the common rail.
However, both the above-described solutions for varying the instantaneous flow rate of the high-pressure pump are mechanically complex and do not allow to adjust the instantaneous flow rate of the high-pressure pump with high accuracy. Furthermore, the flow rate adjustment device comprising a variable section bottleneck presents a small passage section in case of small flow rates and such small passage section determines a high local pressure loss (local load loss) which may compromise the correct operation of an intake valve which adjusts the fuel intake into a pumping chamber of the high-pressure pump.
For this reason, there has been suggested a solution of the type presented in patent application EP1612402A1, which relates to a high-pressure pump comprising a number of pumping elements operated in reciprocating motion by means of corresponding intake and delivery strokes and in which each pumping element is provided with a corresponding intake valve in communication with an intake pipe fed by a low-pressure pump. On the intake pipe there is arranged a shut-off valve controlled in a choppered manner for adjusting the instantaneous fuel flow rate fed to the high-pressure pump; in other words, the shut-off valve is a valve of the open/closed (on/off) type which is driven by modifying the ratio between the opening time and the closing time so as to vary the instantaneous fuel flow rate fed to the high-pressure pump. In this manner, the shut-off valve always displays an efficaciously wide passage section which does not determine an appreciable local pressure loss (local load loss).
The shut-off valve is controlled synchronously with respect to the mechanical actuation of the high-pressure pump (which is performed by a mechanical transmission which receives the motion from the crankshaft) by means of a driving frequency of the shut-off valve having a constant internal synchronization ratio, predetermined according to the pumping frequency of the high-pressure pump (typically, an opening/closing cycle of the shut-off valve is performed for each pumping stroke of the high-pressure pump). It has been observed that there is a rather narrow critical angle at each pumping of the high-pressure pump; if the opening command of the shut-off valve is given at the critical angle, irregularities in the fuel delivery to the high-pressure pump may occur and such delivery irregularities subsequently cause a perturbation of the fuel pressure inside the common rail.
In order to avoid sending the opening command of the shut-off valve at the critical pumping angle of the high-pressure pump, it has been suggested to phase the shut-off valve commands according to the pumpings of the high-pressure pump; however such a solution requires to accurately know the pumping phase of the high-pressure pump (i.e. the mechanical actuation phase of the high-pressure pump) and thus forces to install an angular encoder in the high-pressure pump with a considerable increase of the costs (an angular encoder is a very expensive sensor and is rather cumbersome).
Additionally, it is worth emphasizing that the mechanical transmission actuating the high-pressure pump receives the motion from the crankshaft and thus presents an actuation frequency proportional to the rotation speed of the crankshaft (consequently, by knowing the rotation speed of the crankshaft the actuation frequency of the mechanical transmission which actuates the high pressure pump is immediately known); however, due to construction and assembly limitations, the mechanical transmission which actuates the high-pressure pump cannot guarantee the respect of the predetermined phase with respect to the crankshaft and thus the phase between the mechanical transmission which actuates the high-pressure pump and the crankshaft cannot be know in advance.